One of the outputs of our brainstorming about how to make CI better was
that it is annoying to have to "babysit" pull requests. This PR attempts
to introduce a notification mechanism by which Azure will notify people
on Slack when a build finishes, so they know they need to go and rerun
or merge the corresponding PR.
This commit also changes the existing $Slack.URL variable to
$Slack.team-daml, to make more explicit where the Slack message is being
sent to (Slack works with one token per destination channel). Both
$Slack.URL and $Slack.team-daml are currently defined as the same token
in Azure.
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